FIELD OF THE INVENTION
[0001] The present invention relates to active matrix arrays and elements thereof. In a
particular aspect, the present invention relates to digital microfluidics, and more
specifically to Active Matrix Electro-wetting-On-Dielectric (AM-EWOD). Electro-wetting-On-Dielectric
(EWOD) is a known technique for manipulating droplets of fluid on an array. Active
Matrix EWOD (AM-EWOD) refers to implementation of EWOD in an active matrix array,
for example by using thin film transistors (TFTs). The invention further relates to
methods of driving such a device.
BACKGROUND OF THE INVENTION
[0005] US6565727 further discloses methods for other droplet operations including the splitting and
merging of droplets and the mixing together of droplets of different materials. In
general the voltages required to perform typical droplet operations are relatively
high. Values in the range 20V - 60V are quoted in the prior art (e.g.
US7329545 (Pamula et al., issued February 12, 2008)). The value required depends principally on the technology used to create the insulator
and hydrophobic layers.
[0006] A notable feature of the EWOD actuation mechanism is that the contact angle of the
liquid droplet with the solid surface depends on the square of the actuation voltage;
the sign of the applied voltage is unimportant to first order. It is thus possible
to implement EWOD with either an AC or a DC drive scheme.
[0007] There are several advantages of implementing EWOD with an AC drive scheme. These
advantages include:
Reduced device degradation through life
Improved insulator reliability
Improved droplet dynamics
For these reasons most groups working on EWOD use AC drive schemes with drive frequencies
of typically 10kHz or higher.
[0008] Many modern liquid crystal (LC) displays use an Active Matrix (AM) arrangement whereby
thin-film transistors control the voltage maintained across the liquid crystal layer.
[0009] LC displays generally require that the voltage across the liquid crystal should be
alternated ("inversion") since the application of a DC field has deleterious effects
for the LC material. Most LC inversion schemes operate so as to invert the sign of
the applied LC voltage with each frame of information written to the display. This
is typically a frequency of 50-60Hz.
[0010] "
Ultra-Low Power System-LCD with Pixel Memory Circuit", Matsuda et al., Proceedings
of IDW '09, AMD1-2, describes an LCD with a pixel memory driving scheme. Pixel memory refers to a technology
whereby the data written to the display is held by an SRAM memory cell within the
pixel. The display is thus 1-bit, i.e. it can only display black or white and not
intermediate grey levels. The advantage of such an implementation is that it removes
the requirement to periodically refresh the voltage written to the display and thus
reduces power consumption. In order to effect inversion of the voltage across the
LC layer, an additional inversion circuit is also included in pixel. This enables
the inversion frequency to be higher than the data refresh rate of the display.
[0011] US7163612 (J. Sterling et al.; issued Jan. 16, 2007) describes how TFT based electronics may be used to control the addressing of voltage
pulses to an EWOD array by using circuit arrangements very similar to those employed
in AM display technologies.
[0012] Such an approach may be termed "Active Matrix Electro-wetting on Dielectric" (AM-EWOD).
There are several advantages in using TFT based electronics to control an EWOD array,
namely:
Driver circuits can be integrated onto the AM-EWOD array substrate.
TFT-based electronics are well suited to the AM-EWOD application.
[0013] They are cheap to produce so that relatively large substrate areas can be produced
at relatively low cost
[0014] TFTs fabricated in standard processes can be designed to operate at much higher voltages
than transistors fabricated in standard CMOS processes. This is significant since
many EWOD technologies may require EWOD actuation voltages in excess of 20V to be
applied.
[0015] An alternative technology for implementing droplet microfluidics is Dielectrophoresis
(DEP). Dielectrophoresis is a phenomenon whereby a force may be exerted on a dielectric
particle by subjecting it to a varying electric field.
[0016] Unlike EWOD (which is a contact line phenomenon associated with the properties of
surfaces), DEP is a bulk phenomenon associated with the different polarisabilities
of a dielectric particle and its surrounding medium. "
Integrated circuit/microfluidic chip to programmably trap and move cells and droplets
with dielectrophoresis", Thomas P Hunt et al, Lab Chip, 2008,8,81-87 describes a silicon integrated circuit (IC) backplane to drive a dielectrophoresis
array for digital microfluidics. This reference describes an integrated circuit for
driving AC waveforms to array elements, shown in Figure 1. The circuit consists of
a standard SRAM memory cell 104 to which data can be written and stored, switch circuitry
106, and an output buffer stage 108. According to the operation of the switches (transistors)
110 and 112 either the AC signal Vpix(shown as a 5V 1MHz square wave in this example)
or complementary signal inverse Vpix is written to the pixel. This reference also
gives an overview of the physics of DEP, and describes how the DEP force depends on
the gradient of the electric field squared, the complex permittivities of the particle
and dielectric medium and geometrical factors.
[0017] Lab Chip, 2008, 8, 1325 - 1331 (Fan et al, 28th May 2008) describes how both EWOD and DEP can be implemented in a passive EWOD device to control
both a liquid droplet and also the movement of particles within a droplet. The basic
arrangement of the electrodes in this device is shown in Figure 2. The large, square
electrodes 48 are used to manipulate the position of the liquid droplets using the
EWOD actuation mechanism as is well know. The smaller rectangular electrodes 50 may
then be used for manipulating dielectric particles within the droplet using the DEP
actuation measurement, for example to sort dielectric particles to one side of the
droplet.
[0018] A notable disadvantage of such passive architectures for implementing EWOD and/or
DEP is that a separate electrical connecting wire (e.g. 52) must be made to each individual
electrode. The total number of individually controllable elements within an array
is thus limited by the number of electrical inputs to the device. This makes large
arrays impractical to implement.
SUMMARY OF THE INVENTION
[0019] According to an aspect of the invention, a microfluidic device is provided which
includes a plurality of array elements configured to manipulate one or more droplets
of fluid on an array, each of the array elements including a top substrate electrode
and a drive electrode between which the one or more droplets may be positioned, the
top substrate electrode being formed on a top substrate, and the drive electrode being
formed on a lower substrate; and active matrix drive circuitry arranged to provide
drive signals to the top substrate and drive electrodes of the plurality of array
elements to manipulate the one or more droplets among the plurality of array elements.
With respect to one or more of the array elements the active matrix drive circuitry
is configured to provide the drive signals to the top substrate and drive electrodes
to selectively manipulate the one or more droplets within the array element both by
Electro-wetting-on-Dielectric (EWOD) and by Dielectrophoresis (DEP).
[0020] According to yet another aspect of the invention, a method of driving a microfluidic
device which includes a plurality of array elements configured to manipulate one or
more droplets of fluid on an array is provided. The method includes selectively supplying
by way of active matrix drive circuitry a DC or relatively low frequency AC voltage,
and a relatively high frequency AC voltage, across the one or more droplets within
one or more of the array elements to manipulate the one or more droplets by Electro-wetting-on-Dielectric
(EWOD) and Dielectrophoresis (DEP), respectively.
[0021] To the accomplishment of the foregoing and related ends, the invention, then, comprises
the features hereinafter fully described and particularly pointed out in the claims.
The following description and the annexed drawings set forth in detail certain illustrative
embodiments of the invention. These embodiments are indicative, however, of but a
few of the various ways in which the principles of the invention may be employed.
Other objects, advantages and novel features of the invention will become apparent
from the following detailed description of the invention when considered in conjunction
with the drawings.
BRIEF DESCRIPTION OF DRAWINGS
[0022] In the annexed drawings, like references indicate like parts or features:
Figure 1 shows prior art: the array element circuit of a backplane for control of
droplets by dielectrophoresis;
Figure 2 shows prior art: a passive device architecture for implementing EWOD and
DEP;
Figure 3 shows an AM-EWOD device in schematic perspective in accordance with an exemplary
embodiment of the invention;
Figure 4 shows a cross section through some of the array elements of the device;
Figure 5 shows a schematic illustration of the arrangement of thin film electronics
in the device;
Figure 6 shows a schematic illustration of the array element circuit of a first embodiment;
Figure 7 shows a schematic illustration of the second column driver circuit of a first
embodiment;
Figure 8 shows exemplary timing signals of the drive signals V1A, V1B and V1C in accordance
with a first embodiment;
Figure 9 shows a circuit model representing the electrical characteristics of the
insulator layer, hydrophobic layer and liquid droplet;
Figures 10A - 10C shows the operation of the device of the first embodiment to perform
the operation of particle concentration by DEP;
Figure 11 shows a schematic illustration of the arrangement of thin film electronics
in the device according to a second embodiment of the invention;
Figure 12 shows a schematic illustration of the array element circuit according to
a third embodiment of the invention; and
Figure 13 shows a schematic illustration of the arrangement of thin film electronics
in the device according a fourth embodiment of the invention.
DESCRIPTION OF REFERENCE NUMERALS
[0023]
- 4
- Liquid droplet
- 6
- Contact angle θ
- 16
- Hydrophobic surface
- 20
- Insulator layer
- 26
- Hydrophobic layer
- 28
- Top substrate electrode
- 32
- Spacer
- 34
- Non-ionic liquid
- 36
- Top substrate
- 38
- Electrode
- 40
- Circuit node
- 42
- Electrode array
- 43
- Array element
- 44
- Particle
- 46
- DEP zone
- 48
- Electrode
- 50
- Electrode
- 52
- Wire
- 72
- Lower substrate
- 74
- Thin film electronics
- 76
- Row driver circuit
- 78
- First column driver circuit
- 80
- Serial interface
- 82
- Connecting wires
- 83
- Voltage supply interface
- 84
- Array element circuit
- 86
- Second column driver circuit
- 104
- SRAM memory cell
- 106
- Switch circuitry
- 108
- Output buffer stage
- 110
- Switch transistor
- 112
- Switch transistor
- 203
- Storage capacitor
- 206
- Switch transistor
- 210
- Memory node
- 212
- Inverter
- 214
- First analogue switch
- 216
- Second analogue switch
- 222
- Memory circuit
- 224
- Inversion circuit
- 226
- First inverter
- 228
- Second inverter
- 230
- Switch transistor
- 500
- Element of second column driver circuit
- 502
- First bank of three switches
- 504
- Second bank of three switches
- 506
- Digital output select circuit
- 508
- Digital control circuit
DETAILED DESCRIPTION OF INVENTION
[0024] This invention describes active matrix architectures for manipulating fluidic droplets
by both EWOD and DEP mechanisms. Active matrix array elements may be realized where
liquid droplets can be manipulated by either EWOD or DEP.
[0025] Referring to Figure 3, shown is a droplet microfluidic device in accordance with
an exemplary embodiment of the present invention. The droplet microfluidic device
is an active matrix device with the capability of manipulating fluids by both EWOD
and by DEP. The device is further capable of manipulating droplets by EWOD in one
part of the array and at the same time manipulating droplets by DEP in another part
of the array. The device is also reconfigurable such that a droplet in a given part
of the array can be manipulated by EWOD at one time and by DEP at another time.
[0026] The droplet microfluidic device has a lower substrate 72 with thin film electronics
74 disposed upon the substrate 72. The thin film electronics 74 are arranged to drive
array element electrodes, e.g. 38. A plurality of array element electrodes 38 are
arranged in an electrode array 42, having M x N elements where M and N may be any
number. In the exemplary embodiments herein, M and N are both equal to or greater
than 2. A liquid droplet 4 is enclosed between the substrate 72 and the top substrate
36, although it will be appreciated that multiple droplets 4 can be present without
departing from the scope of the invention. Thus, one or more droplets 4 may be present
within the array. The liquid droplet 4 may also contain one or more particles 44 suspended
within it. The particles 44 may have electrical properties that are different to the
liquid droplet and/or to other particles contained within the liquid droplet 4.
[0027] Figure 4 shows a pair of the array elements in cross section. The device includes
the lower substrate 72 having the thin-film electronics 74 disposed thereon. The uppermost
layer of the lower substrate 72 (which may be considered a part of the thin film electronics
layer 74) is patterned so that a plurality of electrodes 38 (e.g., 38A and 38B in
Figure 4) are realised. These may be termed the EW drive elements. The term EW drive
element may be taken in what follows to refer both to the electrode 38 associated
with a particular array element, and also to the node of an electrical circuit directly
connected to this electrode 38. The droplet 4, consisting of an ionic material is
constrained in a plane between the lower substrate 72 and the top substrate 36. A
suitable gap between the two substrates may be realised by means of a spacer 32, and
a non-ionic liquid 34 (e.g. oil) may be used to occupy the volume not occupied by
the droplet 4. An insulator layer 20 disposed upon the lower substrate 72 separates
the conductive electrodes 38A, 38B from the hydrophobic surface 16 upon which the
droplet 4 sits with a contact angle 6 represented by θ. On the top substrate 36 is
another hydrophobic layer 26 with which the droplet 4 may come into contact. Interposed
between the top substrate 36 and the hydrophobic layer 26 is a top substrate electrode
28. The top substrate electrode 28 in the exemplary embodiment is common to all of
the array elements, although it will be appreciated that each array element or groups
of array elements may have its own top substrate electrode.
[0028] By appropriate design and operation of the thin film electronics 74, different voltages,
termed the EW drive voltages, (e.g. V
T, V
0 and V
00) may be applied to different electrodes (e.g. drive element electrodes 28, 38A and
38B, respectively). The hydrophobicity of the hydrophobic surface 16 can be thus be
controlled, thus facilitating droplet movement in the lateral plane between the two
substrates 72 and 36.
[0029] The arrangement of thin film electronics 74 upon the substrate 72 is shown in Figure
5. Each array element 43 of the electrode array 42 contains an array element circuit
84 for controlling the electrode potential of a corresponding electrode 38. Integrated
row driver 76 and first column driver circuit 78 circuits are also implemented in
thin film electronics to supply control signals to the array element circuits 84.
A second column driver circuit 86 is also implemented to supply control signals to
the array element circuits 84. In the exemplary embodiments, the row driver 76, first
column driver 78, second column driver circuit 86 and array element circuits 84 make
up active matrix drive circuitry for providing drive signals to the top substrate
electrode 28 and drive element electrode 38 of each array element 43.
[0030] A serial interface 80 may also be provided to process a serial input data stream
and write the required voltages to the electrode array 42. A voltage supply interface
83 provides the corresponding supply voltages, top substrate drive voltages, etc.,
as described herein. The number of connecting wires 82 between the array substrate
72 and external drive electronics, power supplies etc. can be made relatively few,
even for large array sizes.
[0031] The array element circuit 84 according to a first embodiment is shown in Figure 6.
The remainder of the AM-EWOD device is of the standard construction previously described.
[0032] Each array element circuit 84 is arranged so as to supply a drive voltage V
EW across the liquid droplet 4 and includes the following components:
A memory circuit 222 including:
A column write line COL (originating from the first column driver circuit 78), which
may be common to array elements within the same column
A row select line ROW (originating from the row driver circuit 76), which may be common
to array elements within the same row A storage capacitor 203
A DC supply voltage Vref
A switch transistor 206
An inversion circuit 224 including:
A first analogue switch 214
A second analogue switch 216
A supply voltage V1 (originating from the second column driver circuit 86) which may be common to array
elements within the same column A second supply voltage V2 (originating from the second column driver circuit 86) which may be common to all
elements within the same column An inverter 212
[0033] The array element circuit 84 is connected as follows:
[0034] The column write line COL is connected to the source of the switch transistor 206.
The row select line ROW is connected to the gate of the switch transistor 206. The
storage capacitor 203 is connected between the DC supply voltage Vref and the drain
of the switch transistor 206. The drain of the switch transistor 206 is connected
to the input of the inverter 212, the gate of the n-type transistor of the first analogue
switch 214 and the gate of the p-type transistor of the second analogue switch 216.
The output of the inverter 212 is connected to the gate of the p-type transistor of
the first analogue switch 214 and to the gate of the n-type transistor of the second
analogue switch 216. The supply voltage V
1 is connected to the input of the first analogue switch 214. The supply voltage V
2 is connected to the input of the second analogue switch 216 and to the top substrate
electrode 28. The outputs of the first analogue switch 214 and the second analogue
switch 216 are each connected to the electrode 38 forming the EW drive electrode.
[0035] The array element circuit 84 operates so as to supply a voltage signal, either V
1 or V
2, to the EW drive electrodes 38 of each array element.
[0036] The operation of the array element circuit 84 is described as follows:
[0037] The array element circuit 84 includes the aforementioned two functional blocks, the
memory circuit 222 and the inversion circuit 224. The memory circuit 222 is a standard
DRAM circuit. A digital write voltage V
WRITE corresponding to either logic "0" or logic "1" state may be written to the memory
by loading the column write line COL with the required voltage and then applying a
high level voltage pulse to the row select line ROW. This turns on the switch transistor
206 and the write voltage is then written to the memory node 210 and stored across
the storage capacitor 203. Due to leakage of the switch transistor 206, the memory
circuit 222 must be re-written to periodically so as to refresh the V
WRITE voltage at the memory node 210.
[0038] In the case where logic "1" state is written to the memory circuit 222, the inversion
circuit 224 becomes configured such that the first analogue switch 214 is turned on,
and the second analogue switch 216 is turned off. As a result supply voltage V
1 is applied to the electrode forming the EW drive element 38. In the case where logic
"0" state is written to the memory function 222, the inversion circuit 224 becomes
configured such that the first analogue switch 214 is turned off, and the second analogue
switch 216 is turned on. In this case supply voltage V
2 is applied to the conductive electrode forming the EW drive electrode 38.
[0039] The supply voltages V
1 and V
2 may be common to array elements within the same column and are supplied by the second
column driver circuit 86, and hence are also referred to herein as column lines V
1 and V
2. A possible circuit arrangement for the second column driver circuit 86 is as follows.
The second column driver circuit 86 may include a number of circuit elements 500,
with one element 500 for each column of the array. Figure 7 shows an exemplary arrangement
of the circuitry of an element 500 of the second column driver circuit 86. Each circuit
element 500 includes:
A first bank of three switches 502
A second bank of three switches 504
A digital output select circuit 506
A digital control circuit 508
[0040] The digital control circuit 508 has an external clock input CK and an output Q which
connects to a clock input CKA of the digital output select circuit 506. The digital
output select circuit 506 has a two bit data input DATA1 and DATA2 and a three bit
parallel data output O1 and O2 and 03. The three outputs O1, O2 and 03 of the digital
output select circuit 506 are used to control the three switches in the banks of switches
502 and 504. The outputs of the three switches in bank 502 are connected together
and are connected to the output V
1 representing the supply voltage which is provided to the first analogue swtich 214
in the respective array elements. The outputs of the three switches in bank 504 are
connected together and are connected to the output V
2 representing the supply voltage which is provided to the second analogue switch 216
in the respective array elements and to the top substrate electrode 28. The inputs
V1A, V1B and V1C comprise the three inputs to the bank of three switches 502, and
the inputs V2A, V2B and V2C comprise the three inputs to the bank of three switches
504. These inputs may be global signals externally supplied to the device.
[0041] The operation of second column driver circuit element 500 is described as follows.
The second column driver circuit element 500 essentially functions so as to switch
one of the three input signals V1A, V1B or V1C to the output V
1 which is then input to the array elements within that particular column of the array.
Similarly, V2A, V2B or V2C is switched to the output V
2, such that when V
1=V1A then also V
2=V2A and when V
1=V1B then also V
2=V2B, and when V
1=V1C then also V
2=V2C.
[0042] The choice of input signal switched to V
1 is determined by the digital word on inputs DATA1 and DATA2, for example DATA1=0,
DATA2=0 may be configured to switch V1A to V
1, DATA1=0, DATA2=1 may be configured to switch V1B to V
1 and DATA1=1, DATA2=0 may be configured to switch V1C to V
1. The digital output select circuit 506 may be implemented with standard logic elements
(e.g. inverters and NAND gates) using standard means as is very well known. The digital
control circuit 508 may be implemented as a shift register element, for example using
flip-flops and latches such that the logic level of input D is sampled to the output
Q on the rising edge of clock input CK. The output Q in turn is input to the clock
input CKA of the digital output select circuit 506 so that the logic levels of DATA1
and DATA2 are sampled to the outputs O1, O2 and O3. The values of DATA1, DATA2, D
and CK are provided to the second column driver circuit elements 500 from an external
source via the serial interface 80, for example, to carry out EWOD or DEP manipulation
in the corresponding array elements as desired.
[0043] The second column driver element circuit 500 thus operates to determine the signal
switched to the column line V1 for each column of the array, i.e. whether V1A, V1B
or V1C. Likewise it also determines the signal switched to the column line V2 for
each column of the array, i.e. whether V2A, V2B or V2C.
[0044] Figure 8 shows the time dependence of the waveforms of supply voltages V1A, V1B and
V1C. V1A is a DC signal at logic high level. V1B is a square-wave of relatively low
frequency f
1. In this context, relatively low frequency may mean a frequency chosen such that
the electro-wetting force is large and the liquid droplet may be manipulated by EWOD.
For example 10kHz could be used, or 1 kHz or 100Hz.
[0045] V1 C is a square-wave of relatively high frequency f
2. The frequency of f
2 may be chosen so as to be suitable so as to manipulate particles within the liquid
droplet by DEP. Suitable values for f
2 will therefore depend on the dielectric properties of both the liquid droplet 4 and
the particles to be manipulated. A suitable frequency for f
2 is likely to be typically within the range 100kHz to 10MHz, and could for example
be 1 MHz.
[0046] V2A, V2B and V2C (not shown) are the logical complement of these signals. V1A, V1B
and V1C may be global signals, generated and supplied for example by an external PCB.
[0047] It may be noted that the frequencies f
1 and f
2 may therefore be externally controllable and may be determined and set according
to the requirements of operation and further description as to their values will shortly
be provided.
[0048] The overall function of the thin film electronics 74 is thus summarized as follows.
Each array element circuit 84 contains a programmable 1-bit memory function which
may be programmed by row driver circuit 76 and column driver circuit circuit 78 by
way of write voltages based upon which determines whether signal V
1 or V
2 is written to the EW drive electrode 38. The second column driver circuit 86 is used
to determine which one of three global input signals V1A, V1B and V1C are written
to the V
1 signal of a given column in the array, and likewise which of V2A, V2B and V2C to
the V
2 signal of a given column in the array.
[0049] The signals V1A, V1B and V1C are arranged to actuate the droplet 4 by DC electro-wetting,
AC electro-wetting and DEP respectively. These functions may be explained with reference
to Figure 9, which shows a circuit model representing the electrical characteristics
of the insulator layer 20, hydrophobic layer 16 and liquid droplet 4, between the
drive electrode 38 and the top substrate electrode 28 and when a liquid droplet 4
is present at that particular location within the array. For simplicity of description
the effect of any particles suspended within the liquid droplet 4 are not here considered.
The liquid droplet 4 may be represented as capacitor C
DROP in parallel with a resistor R
DROP. The circuit node labeled 40 corresponds to the interface between the droplet 4 and
the hydrophobic surface 16. The combination of the insulator layer 20 and the hydrophobic
layer 16 may be represented electrically as a capacitor Ci. DC electro-wetting, AC
electro-wetting and DEP are performed as follows:
[0050] To perform DC electro-wetting, the voltage supply V2A is connected to the top substrate
electrode 28. The array element circuit 84 at a given location may be programmed so
that either V1A or V2A is connected to the drive electrode 38. If the V1A is written
to the drive electrode 38, a potential difference equal to the DC potential V1A -
V2A is maintained between the electrode 38 and the top substrate electrode 28. With
reference to Figure 9, this DC potential is dropped wholly across capacitor Ci resulting
in an electric field at the interface between the liquid droplet 4 and the hydrophobic
layer 16. This therefore acts to change the contact angle by the mechanism of electro-wetting
as is well known and described in prior art references. On the other hand if V2A is
written to the drive electrode, the potential difference between the electrode 38
and the top substrate electrode 28 is zero and there is no electric field at the interface
between the liquid droplet 4 and the hydrophobic layer 16. By suitable choice of signals
supplied to different array elements, the positions of liquid droplets 4 in the array
may thus be manipulated as is also well known and described in the prior art.
[0051] To perform AC electro-wetting, the voltage supply V2B is connected to the top substrate
electrode 28. The array element circuit 84 at a given array element may be programmed
so that either V1B or V2B is connected to the drive electrode 38. If the V1B is written
to the drive electrode, a potential difference equal to the DC potential V1B - V2B
is maintained between the electrode 38 and the top substrate electrode 28. This corresponds
to an AC potential of frequency f
1. The appropriate choice of frequency f
1 for controlling the liquid droplet 4 by AC electro-wetting may be explained with
reference to Figure 9. The electro-wetting force depends on the electric field at
the interface between the hydrophobic layer 16 and the liquid droplet 4, and so the
general requirement is that most of the applied voltage must be dropped across capacitor
Ci and not through the droplet 4 itself. The choice of frequency may depend on the
resistance R
DROP but is typically quite low, for example 10kHz. In this low frequency case therefore,
an AC electric field is present at the interface between the hydrophobic layer 16
and the liquid droplet 4 (node 40 in Figure 9) such that the droplet is manipulated
by AC electro-wetting as is well known. In the other case whereby the array element
circuit 84 is programmed so that V2A is written to the drive electrode, the potential
difference between the electrode 38 and the top substrate electrode 28 is zero and
no electro-wetting force results. Again, by suitable choice of signals supplied to
different array elements, the positions of liquid droplets in the array may thus be
manipulated by electro-wetting as is also well known and has been described in the
prior art.
[0052] To perform DEP, the voltage supply V2A is connected to the top substrate electrode
28. The array element circuit 84 at a given location may be programmed so that either
V1C or V2C is connected to the drive electrode 38. In both cases the result is that
a time varying electric field is applied between electrode 38 and top substrate electrode
28. In order to implement DEP, the frequency f
2 of the voltage pulses V1C and V2C must be sufficiently large such that most of the
voltage difference between the electrode 38 and the top substrate electrode 28 is
dropped across the liquid droplet 4. The net result is a time varying electric field
in the body of the liquid droplet 4 which may be used to manipulate dielectric particles
44 (which, for example, could be beads or cells) contained within the liquid droplet
4. The exact choice of the frequency f
2 is a function of the real and imaginary components of the permittivities of the liquid
droplet medium and of the particles within the droplet being manipulated. This frequency
dependency is described by the Claussius-Mossotti factor which is described in prior
art references to the DEP phenomenon and which is very well known. Essentially f
2 may be chosen so that either the particles are more polarisable than the dielectric
medium of the liquid droplet 4 (positive DEP) or such that the particles 44 are much
less polarisable than the dielectric medium of the liquid droplet (negative DEP).
The choice of frequency and whether to use positive or negative DEP depend to a large
extent on the dielectric properties of the liquid droplet 4 and the particles 44 being
manipulated. By suitable choice of signals supplied to different array elements, DEP
may be used to manipulate the positions of particles within a liquid droplet. In an
alternative implementation, DEP may be performed with signal V2B supplied to the top
electrode. The operation would be essentially the same, except that the polarity of
the V1C and V2C signals would need to be inverted at the times when V2B was at logic
high level. This would be in order to ensure that the magnitude of the electric field
in the device is the same when V2B is at the high level as when it is at the low level.
[0053] An example of how DEP and EWOD may be used to manipulate particles within the liquid
droplet is shown in Figures 10A - 10C. The figures show a cross section of a device,
as in Figure 4, and where three drive element electrodes 38A, 38B and 38C are shown.
The liquid droplet 4 contains a number of particles 44. Figures 10A - 10C show how
the device may use DEP to perform the operation of concentrating particles within
a liquid droplet, Figures 10A, 10B and 10C showing successive situations of the operation
in time. By modifying the signals applied over time to electrodes 38A, 38B and 38C,
the application of the electric field may be used to move dielectric particles 44
to one side of the liquid droplet 4 ((e.g., to the right as shown Figure 10C). At
a still later time, the actuation mechanism may be switched to DC EWOD which may be
used to split the droplet 4. The overall effect may therefore be used to concentrate
the particles 44 within the liquid droplet 4. This could be used for example in a
washing operation, whereby liquid surrounding particles 44 has excess reagent washed
away. Another example application is in cell or bead concentration as may be used
in a number of well known chemical or biochemical assays.
[0054] It should be noted that the actuation mechanisms for EWOD and DEP are physically
completely different. Electro-wetting is essentially a surface phenomenon, and the
strength of the electro-wetting force depends on the magnitude of the electric field
at the interface between the liquid droplet 4 and the hydrophobic layer 16. DEP on
the other hand is a bulk phenomenon, and the DEP force depends on the application
of a time varying E-field through the bulk of the liquid droplet. It has however been
realised that, despite the different actuation mechanisms, the drive requirements
for EWOD and DEP are similar and that switching between one and the other may be effected
just by changing the frequency of the applied voltage waveform between electrode 38
and top substrate electrode 28.
[0055] An advantage of this embodiment is that by facilitating both EWOD and DEP mechanisms
within the same active matrix device, both liquid droplets 4 and dielectric particles
44 suspended within them can be independently controlled. This can have application
in particle concentration, washing, particle sorting etc. which are useful or necessary
steps in a number of applications of AM-EWOD technology, for example immunoassays
for Point of Care diagnostics. The availability of both actuation measurements in
an active matrix device facilitates the creation of large format fully reconfigurable
devices that are capable of performing complex digital microfluidic operations and
of performing a number of operations in parallel within the same device.
[0056] A further advantage is that the arrangement described is fully reconfigurable. In
particular, any given column of the array can be arranged to perform either EWOD or
DEP at a given point in time, and that different columns may be used to perform different
functions simultaneously. In other words, EWOD or DEP actuation may be selected individually
for each column.
[0057] A further advantage of the circuit implementation described is that the number of
electronic components within the array element circuit is relatively small. The same
number of transistors are used in the array element circuit 84 of this embodiment
as were used to perform just a DEP function as was shown in Figure 1 and described
in the background art section.
[0058] A further advantage of this embodiment is that the frequencies f
1 and f
2, associated with actuation by EWOD and DEP respectively, are entirely flexible and
controllable independent of other timing parameters associated with the operation
of the device. This is advantageous since the optimum operating frequency will be
dependent on the properties of the medium of the liquid droplet 4 and of any dielectric
particles 44 it contains.
[0059] A further advantage is that the high and low voltage levels of V1A, V1B and V1 C
(and their logic complements) may be adjusted to control the strength of the EWOD
and DEP actuation mechanisms and it will be appreciated that the voltage levels required
for implementing DEP and EWOD do not necessarily need to be the same. This advantage
may be important, for example, to avoid excessive power consumption when performing
DEP at high frequency.
[0060] Based on the disclosure herein, it will be obvious to one skilled in the art that
a number of possible variants of the above embodiment could be implemented. For example,
the signals V
1 and V
2 supplied to the array elements could be arranged to be common to elements within
the same row instead of elements within the same column. In this manner, EWOD or DEP
actuation may be selected individually for each row. Another possible variant would
be for the second column driver 86 to select signals V
1 and V
2 based on just two possibilities, e.g. V1A and V1C as previously described and corresponding
to the cases of options of actuating the liquid droplet 4 by either DC EWOD or by
DEP.
[0061] A second embodiment of the invention is as the first embodiment, where the array
elements 43 are arranged so as to have a large aspect ratio, being significantly longer
in the column direction than in the row direction. A large aspect ratio may for example
be realised as array elements 43 whose long dimension exceeds the short dimension
by a factor of 2, or a factor of 5, or a factor of 10 or a factor of 20, or a factor
of 50. The arrangement of thin film electronics 74 according to this embodiment is
shown in Figure 11. According to this embodiment, the circuit schematics may be identical
to the first embodiment, and the large aspect ratio may be realized by known layout
techniques. By making the array element 43 smaller in the row direction, and by applying
alternate DEP driving signals to array elements in adjacent columns (for example,
V1C may be applied to the electrode 38 of the array elements in column Z and V2C may
be applied to the electrodes 38 of the array elements in column Z+1), the gradient
of the electric field and hence the strength of the DEP force is increased. According
to the operation of this embodiment, DEP may preferentially be used to manipulate
dielectric particles 44 within the liquid droplet 4 in the row direction, the maximum
DEP force available being in the direction of the shorter length of array element
43. An advantage of this embodiment is that by maximizing the DEP force available,
the efficacy of DEP operations, e.g. particle sorting is increased. A further advantage
is that by maximizing the DEP actuation force in this way by means of the geometry,
it may be possible to reduce the voltage amplitude of the signals V1C and V2C whilst
still achieving sufficient DEP force. This will be advantageous for reducing power
consumption etc.
[0062] It will be apparent to one skilled in the art that in a modification to this embodiment,
the shorter dimension of the array element 43 could also be arranged to be in the
column direction, with operation similar to as has already been described.
[0063] A third embodiment of the invention is as the first embodiment, whereby an alternative
array element circuit 84a is used, as is shown in Figure 12. The remainder of the
AM-EWOD device is of the standard construction previously described and includes a
top substrate 36 having a top substrate electrode 28. The array element circuit 84a
according to this embodiment performs the same function as that for the first embodiment,
namely to supply one of voltage lines V1 or V2 to the electrode 38, thus controlling
the voltage signal supplied between this electrode and the top substrate electrode
28, and so across the liquid droplet 4.
[0064] The array element circuit 84a in this embodiment contains the following elements:
A memory circuit 222a including:
A column write line COL (originating from the first column driver circuit 78), which
may be common to array elements within the same column
A row select line ROW (originating from the row driver circuit 76), which may be common
to array elements within the same row
An n-type switch transistor 206
A p-type switch transistor 230
A first inverter 226
A second inverter 228
An inversion circuit 224a including:
A first analogue switch 214
A second analogue switch 216
A voltage supply V1, which may be common to all elements within the array
A second voltage V2, which may be common to all elements within the array
[0065] The circuit 84a is connected as follows:
The column write line COL is connected to the source of the switch transistor 206.
The row select line ROW is connected to the gate of the switch transistor 206 and
the gate of the switch transistor 230. The drain of the switch transistor 230 is connected
to the drain of the switch transistor 206 and to the input of the first inverter 226.
The output of the first inverter 226 is connected to the input of the second inverter
228, the gate of the p-type transistor of the first analogue switch 214 and the gate
of the n-type transistor of the second analogue switch 216. The output of the second
inverter 228 is connected to the gate of the n-type transistor of the first analogue
switch 214 and to the gate of the p -type transistor of the second analogue switch
216 and to the source of the switch transistor 230. The voltage supply V1 is connected to the input of the first analogue switch 214. The voltage V2 is connected to the input of the second analogue switch 216 and to the top substrate
electrode 28. The outputs of the first analogue switch 214 and the second analogue
switch 216 are each connected to the conductive electrode 38 forming the EW drive
electrode.
[0066] The operation of this embodiment is similar to the first embodiment. The memory function
is written by applying a high voltage pulse to the row select line ROW so as to turn
the switch transistors 206 and 230 on. The voltage on the column write line COL is
then written to the memory node 210. The operation of the inversion circuit 224a is
as described for the first embodiment, with the exception that the inverted memory
node signal can be obtained from connection to the node between the two inverters
226,228.
[0067] An advantage of this embodiment is that the SRAM memory structure of the memory circuit
222a does not require continual refresh. This may facilitate device operation with
lower power consumption than is possible with a DRAM memory function. The array element
circuit 84a is implemented with a minimal number of active components (ten transistors)
thus minimizing layout footprint and maximizing manufacturing yield.
[0068] A fourth embodiment of the invention is as any of the previous embodiments where
the dual EWOD / DEP function is restricted to just a part of the active matrix device.
An example implementation of the thin film electronics in this embodiment is shown
in Figure 13. In this implementation, columns 1 to X, shown in the figure on the left
hand side of the array, are designed to manipulate the liquid droplet solely using
EWOD; the signals V
1 and V
2 supplied to these columns may be hardwired to supply lines, e.g. V1A and V1B, and
are not required to be switchable between different driving options. Columns 1 to
X would therefore be used just for the purpose of manipulating droplets by EWOD. Columns
X+1 to Y of the array may be of an architecture as previously described, having a
second column driver circuit 86, such that the signal lines V
1 and V
2 supplied to array elements within these columns can take options V1A, V1B or V1C,
etc., as previously described for the first embodiment. Therefore in these columns
of the array, liquid droplets 4 can be manipulated by EWOD or by DEP as previously
described. This embodiment thus describes an AM-EWOD device having a dedicated "DEP
zone" 46 (i.e. columns X+1 to Y) where DEP may also be performed. In operation of
the device, in order to perform DEP, a droplet would be moved by means of electro-wetting
to a location somewhere in the DEP zone 46. The required DEP operation, e.g. cell
sorting, could be performed before then moving the droplet 4 to a different location
in the array, for example as may be required for the next stage of the assay being
performed.
[0069] According to an aspect of the invention, active matrix drive circuitry is arranged
such that the actuation mechanism (EWOD or DEP) can be selected individually for each
column in the array at any particular time.
[0070] According to a further aspect of the invention, a method of driving is disclosed
whereby the drive signals applied across a liquid droplet can be selected to be either
a DC or low frequency AC voltage waveform for actuating the droplet by EWOD, or else
a high frequency AC voltage waveform for actuating the droplet by DEP.
[0071] In accordance with another aspect, the active matrix drive circuitry is operative
to render the device reconfigurable in that the one or more droplets in a given part
of the array can be manipulated by EWOD at one time and by DEP at another time.
[0072] In accordance with still another aspect, the active matrix drive circuitry is configured
to selectively provide the drive signals the top substrate and drive electrodes of
the one or more array elements in order to be either a DC or relatively low frequency
AC voltage waveform across the one or more liquid droplets to manipulate the one or
more liquid droplets by EWOD, or a relatively high frequency AC voltage waveform for
manipulating the one or more droplets by DEP.
[0073] According to yet another aspect, the one or more array elements are located in different
parts of the array, and the active matrix drive circuitry is configured so as to be
capable of manipulating at the same time some of the one or more droplets in one of
the parts by EWOD and others of the one or more droplets in another of the parts by
DEP.
[0074] In yet another aspect, the array elements are arranged in an MxN array, where M and
N are both equal to or greater than 2.
[0075] According to another aspect, the active matrix drive circuitry is configured in order
that EWOD or DEP actuation may be selected individually for each column in the array
at any particular time.
[0076] In accordance with another aspect, the active matrix circuitry includes a row driver
circuit configured to select rows within the array, a first column driver circuit
for providing write voltages to the array elements within a given row when selected,
and a second column driver circuit for providing the drive signals to the top substrate
electrodes and drive electrodes of respective columns within the array to selectively
manipulate the one or more droplets within the respective columns by EWOD or DEP based
on the write voltages provided to the array elements within the respective columns.
[0077] According to another aspect, the active matrix circuitry is configured with respect
to some columns in the array to manipulate the one or more droplets solely using EWOD,
and with respect to other columns in the array to manipulate the one or more droplets
selectively by EWOD or DEP.
[0078] In still another aspect, the active matrix drive circuitry is configured in order
that EWOD or DEP actuation may be selected individually for each row in the array
at any particular time.
[0079] According to another aspect, the drive signals include at least one of DC signal
and a relatively low frequency signal, a relatively high frequency signal, and logical
complements thereof.
[0080] With still another aspect, the DC signal and its logical complement are selectively
applied to the top substrate electrode and the drive electrode of a given array element
to manipulate the one or more droplets therein by DC EWOD.
[0081] In accordance with another aspect, the relatively low frequency signal and its logical
complement are selectively applied to the top substrate electrode and the drive electrode
of a given array element to manipulate the one or more droplets therein by AC EWOD.
[0082] According to still another aspect, the relatively high frequency signal and its logical
complement are selectively applied to the top substrate electrode and the drive electrode
of a given array element to manipulate the one or more droplets therein by DEP.
[0083] In accordance with another aspect, the array elements have a large aspect ratio.
[0084] According to still another aspect, the array elements share a common top substrate
electrode.
[0085] In accordance with another aspect, the method includes manipulating the one or more
droplets in a given part of the array by EWOD at one time and by DEP at another time.
[0086] According to another aspect, the method includes manipulating at the same time some
of the one or more droplets in one part of the array by EWOD and others of the one
or more droplets in another part by DEP.
[0087] In accordance with still another aspect, the array elements are arranged in an MxN
array, where M and N are both equal to or greater than 2, and the active matrix drive
circuitry is configured in order that EWOD or DEP actuation may be selected individually
for each column in the array at any particular time.
[0088] According to yet another aspect, the active matrix circuitry includes a row driver
circuit configured to select rows within the array, a first column driver circuit
for providing write voltages to the array elements within a given row when selected,
and a second column driver circuit for providing the drive signals to the respective
columns within the array to selectively manipulate the one or more droplets within
the respective columns by EWOD or DEP based on the write voltages provided to the
array elements within the respective columns.
[0089] Advantages of the invention include that large format, reconfigurable AM-EWOD devices
can be realized with additional DEP functionality. The addition of DEP as a second
actuation mechanism enables dielectric particles (e.g. cells, beads) within a liquid
droplet to be manipulated separately to the body of the liquid droplet. This may be
used for applications such as sample washing and cell concentration.
[0090] Although the invention has been shown and described with respect to a certain embodiment
or embodiments, equivalent alterations and modifications may occur to others skilled
in the art upon the reading and understanding of this specification and the annexed
drawings. In particular regard to the various functions performed by the above described
elements (components, assemblies, devices, compositions, etc.), the terms (including
a reference to a "means") used to describe such elements are intended to correspond,
unless otherwise indicated, to any element which performs the specified function of
the described element (i.e., that is functionally equivalent), even though not structurally
equivalent to the disclosed structure which performs the function in the herein exemplary
embodiment or embodiments of the invention. In addition, while a particular feature
of the invention may have been described above with respect to only one or more of
several embodiments, such feature may be combined with one or more other features
of the other embodiments, as may be desired and advantageous for any given or particular
application.
INDUSTRIAL APPLICABILITY
[0091] The AM-EWOD device could form a part of a lab-on-a-chip system. Such devices could
be used in manipulating, reacting and sensing chemical, biochemical or physiological
materials. Applications include healthcare diagnostic testing, chemical or biochemical
material synthesis, proteomics, tools for research in life sciences and forensic science.
1. A microfluidic device, comprising:
a plurality of array elements (43) configured to manipulate one or more droplets of
fluid (4) on an array (42), each of the array elements including a top substrate electrode
(28) and a drive electrode (38) between which the one or more droplets (4) may be
positioned, the top substrate electrode (28) being formed on a top substrate (36),
and the drive electrode (38) being formed on a lower substrate (72); and
active matrix drive circuitry (76,78,84,86) arranged to provide drive signals to the
top substrate (28) and drive electrodes (38) of the plurality of array elements (43)
to manipulate the one or more droplets (4) among the plurality of array elements (43),
wherein with respect to one or more of the array elements (43) the active matrix drive
circuitry (76,78,84,86) is configured to provide the drive signals to the top substrate
and drive electrodes (28,38) to selectively manipulate the one or more droplets (4)
within the array element both by Electro-wetting-on-Dielectric (EWOD) and by Dielectrophoresis
(DEP).
2. The device according to claim 1, wherein the active matrix drive circuitry (76,78,84,86)
is operative to render the device reconfigurable in that the one or more droplets
(4) in a given part of the array (42) can be manipulated by EWOD at one time and by
DEP at another time.
3. The device according to any one of claims 1-2, wherein the active matrix drive circuity
(76,78,84,86) is configured to selectively provide the drive signals the top substrate
and drive electrodes (28,38) of the one or more array elements (43) in order to be
either a DC or relatively low frequency AC voltage waveform across the one or more
liquid droplets (4) to manipulate the one or more liquid droplets (4) by EWOD, or
a relatively high frequency AC voltage waveform for manipulating the one or more droplets
(4) by DEP.
4. The device according to any one of claims 1-3, wherein the one or more array elements
(43) are located in different parts of the array (42), and the active matrix drive
circuitry (76,78,84,86) is configured so as to be capable of manipulating at the same
time some of the one or more droplets (4) in one of the parts by EWOD and others of
the one or more droplets in another of the parts by DEP.
5. The device according to any one of claims 1-4, wherein the array elements (43) are
arranged in an MxN array, where M and N are both equal to or greater than 2.
6. The device according to claim 5, wherein the active matrix drive circuitry (76,78,84,86)
is configured in order that EWOD or DEP actuation may be selected individually for
each column in the array (42) at any particular time.
7. The device according to any one of claims 5-6, wherein the active matrix circuitry
(76,78,84,86) includes a row driver circuit (76) configured to select rows within
the array, a first column driver circuit (78) for providing write voltages to the
array elements (43) within a given row when selected, and a second column driver circuit
(86) for providing the drive signals to the top substrate electrodes (28) and drive
electrodes (38) of respective columns within the array to selectively manipulate the
one or more droplets (4) within the respective columns by EWOD or DEP based on the
write voltages provided to the array elements within the respective columns.
8. The device according to any one of claims 5-7, wherein the active matrix circuitry
(76,78,84,86) is configured with respect to some columns in the array to manipulate
the one or more droplets solely using EWOD, and with respect to other columns in the
array to manipulate the one or more droplets selectively by EWOD or DEP.
9. The device according to claim 5, wherein the active matrix drive circuitry (76,78,84,86)
is configured in order that EWOD or DEP actuation may be selected individually for
each row in the array at any particular time.
10. The device according to any one of claims 1-9, wherein the drive signals include at
least one of DC signal and a relatively low frequency signal, a relatively high frequency
signal, and logical complements thereof.
11. The device according to claim 10, wherein the DC signal and its logical complement
are selectively applied to the top substrate electrode (28) and the drive electrode
(38) of a given array element (43) to manipulate the one or more droplets therein
by DC EWOD.
12. The device according to any one of claims 10-11, wherein the relatively low frequency
signal and its logical complement are selectively applied to the top substrate electrode
(28) and the drive electrode (38) of a given array element to manipulate the one or
more droplets therein by AC EWOD.
13. The device according to any one of claims 10-12, wherein the relatively high frequency
signal and its logical complement are selectively applied to the top substrate electrode
(28) and the drive electrode (38) of a given array element to manipulate the one or
more droplets therein by DEP.
14. A method of driving a microfluidic device which includes a plurality of array elements
configured to manipulate one or more droplets of fluid on an array, comprising:
selectively supplying by way of active matrix drive circuitry a DC or relatively low
frequency AC voltage, and a relatively high frequency AC voltage, across the one or
more droplets within one or more of the array elements to manipulate the one or more
droplets by Electro-wetting-on-Dielectric (EWOD) and Dielectrophoresis (DEP), respectively.
15. The method according to claim 14, comprising manipulating the one or more droplets
in a given part of the array by EWOD at one time and by DEP at another time and/or
manipulating at the same time some of the one or more droplets in one part of the
array by EWOD and others of the one or more droplets in another part by DEP.